Alkali metal phosphate coating solutions and the method of forming coatings therewith



ilnited States Patent M ALKALI METAL PHOSPHATE COATING SOLU- TIGNS AND TIE METHOD OF FORMING COAT- INGS THEREWITH Hellmut Ley, Waiter Stenger, Willy Werner, and Karl Lampatzer, Frankfurt am Main, Germany, assignors to Parker Rust Proof Company, Detroit, Mich, a corporation of Michigan No Drawing. Application June 26, 1952, Serial No. 295,843

Claims priority, application Germany June 28, 1951 8 Claims. (Cl. 1486.'15)

The present invention relates to the solution for and the method of forming phosphate coatings on metallic surfaces. More particularly the invention relates to improved alkali metal phosphate solutions which substantially eliminate the dust-forming tendencies.

Acidic phosphates of the alkali metals, such as sodium phosphate, potassium phosphate or ammonium phosphate, reduce the amount of sludge or insoluble metallic phosphate sediment which is formed, to a negligible quantity in comparison to that formed by the coating-phosphates typified by zinc or manganese phosphates. Alkali metal phosphates in combination with certain oxidizing or accelerating agents produce a highly desirable coating as a base for paint as is taught in the copending application of Robert C. Gibson, U. S. Serial No. 787,621, now U. S. Patent No. 2,609,308, issued September 2, 1952. In the operation of alkali metal phosphate solutions, including ammonium phosphate, ferrous or ferric iron tends to build up in the solution in a relatively minor quantity which is sufiicient to cause the formation of spotty coatings. By spotty coatings is meant that within the coating there are areas in which the coating is concentrated and heavily built up while in other areas the coating is relatively light. Spotty coatings are known to occur only after an alkali metal phosphate solution has been used for a period of continuous operation.

The principal object of this invention is to provide an alkali metal phosphate solution which does not form spotty coatings and which substantially eliminates the formation of dusty coatings on the surfaces of metals such as iron and steel.

Another object is to provide an alkali metal phosphate solution which when applied to surfaces of zinc and alurninum in continuous production greatly reduces the formation of sludge in the processing solution.

A further object is the provision of a method for treating the surfaces of iron, steel, zinc and aluminum with an improved alkali phosphate solution to form coatings which are substantially free of spots and dust.

In accordance with this invention it has been found that the above enumerated difficulties may be overcome and the objects realizedby incorporating in the solution small quantities of certain materials which are capable of sequestering or forming complexes with the ions of the metal being treated.

Alkali metal phosphate solutions form aninsoluble metallic coating on the surface of the metal being treated through the mechanism of inter-reaction of and dissolution of small quantities of the metal of the surface in the contacting solution and immediate precipitation of the coating at the point of metal dissolution. The coating formed is highly adherent and uniform in thickness unless an excess quantity of metal ions from the surface build up in the processing solution. Concentrations of coating occurring in pinpoint locations on the surface, are detrimental to present is increased to within the range of about 3 to about 2,758,949 Patented Aug. 14, 1956 the subsequent application of paints, lacquers or the like. By the incorporation of the sequestering agents of this invention, it is possible either to avoid the formation of spots and dust on the coating completely or to deter the formation of spots or dust sufiiciently so that the coating is uniform, and any dust which is formed occurs as a cloudiness in the solution and gradually settles harmlessly to the bottom of the treating container.

Alkali metal phosphate solutions which are improved by metal sequestering agents in accordance with this invention include the aqueous acidic phosphates of sodium, potassium and ammonium. Such solutions may contain, and preferably do contain, an oxidizing agent. Any of the oxidizing agents which are known to be beneficial in alkali metal phosphate solutions may be employed. Typical of the oxidizing agents which may be used are the chlorates, bromates, sulfites, nitrites, hydrogen peroxide, chlorite and organic nitro compounds. Specific organic nitro compounds that are useful are, for example, m-nitrobenzene sulfonic acid, o-nitro-chlorobenzene-p-sulfonic acid, 3-nitrophthalic acid, and m-nitrobenzoic acid.

A preferred solution may contain proportions of ingredients as follows:

For the purpose of decreasing the chemical reactivity of the stronger solutions of this invention so that they may be transported in stainless steel drums or other metallic containers in conventional use it is desirable to add small quantities of sodium nitrate, for example, an amount which when diluted represents about .3 to 2.0 grams/liter of N03. The sodium oxide concentration results from the use of a sodium salt such as sodium carbonate or sodium hydroxide to regulate the pH of the operating solution preferably within the range of about 3 to about 5. Solutions having higher pH values up to a pH of about 6 are also benefited by the addition agents of this invention.

The materials or addition agents which serve to improve the uniformity of distribution of coating and to substantially eliminate the formation of spots and dust from the above described types of solutions comprise the polycarboxylic alpha amino acids, such as, ethylene diamine tetra acetic acid, triglycine (the triacetic substitution product of ammonia) and the tetra acetic acid derivative of trimethylenediamine. The alkali metal salts of such acids including the di, tri and tetra sodium and potassium salts are particularly suitable for both original make-up and replenishing of the solution during use. Other materials which have been found to function in a manner similar to the polycarboxylic alpha amino acids are the phosphates which are less hydrous than ortho phosphate, namely the polyphosphates such as the pyrophosphates, metaphosphates, tripolyphosphate and including specifically tetra sodium pyrophosphate, sodium metaphosphate, sodium hexametaphosphate, etc. Preferred concentrations for addition to the above preferred solution are from about 1 to about 5 grams/liter of tetra sodium ethylenediamine tetra-acetate, and from about 1 to about 10 grams/ liter of a polyphosphate. It will be understood that the equivalent amount of other salts of ethylene diamine. tetra-acetic acid or the acid itself as well as its other forms may be used equally as well as the tetra sodium salt. Thus the equivalent of about .77 grams to about 3.85 grams/liter of ethylene diamine tetra-acetic acid may be used. When proportions of polyphosphate approaching the upper limit are used, the proportion of sodium oxide 25 grams/ liter.

While most applications do not require the presence of quantities greater thanabout grams/ literof tetra sodium ethylene-diamine tetra-acetate or about grams/liter of a polyphosphate, it is sometimes desirable to employ quantitles of about 1 gram/liter to 10 grams/liter of the'tetra sodium salt of ethylenediamine tetra-acetic acid or its equivalent -or about 1 gram/liter to about'20 grams/liter of apolyphosphate.

The solutions of this invention may be applied to the metal being treated either by spraying or by dipping the article into the solution. For convenience it is preferred to incorporate the addition agents of this invention in the original make-up material or replenishing material whether in solid or concentrated solution form, and to regulate the pH to the desired value as the make-up is 'diluted to operating solution strength. If desired, however, the addition agent may be added "separately when the'solution begins to appear cloudy.

Typical'make-up and replenishing materials in the form of solid salts may have the following ingredients:

Make-up 1 Monosodium phosphate "grams" 7 to 40 Sodium pyrophosphate d0 2-to Sodium chlorate do.. 3 to 25 If desired, a smal amount of sodium nitrate may also be present, for example, 0.4 to 3.0 grams.

Make-up 2 Monosodium phosphate ..grams 7 to 40 Tetra sodium ethylenediamine tetra-acetate grams 2 to 15 Sodium'chlorate do 3 to 25 It will be appreciated that the formulations of Make-up 1 and 2 are illustrative only and that other alkali phosphates, polyphosphates and polycarboxylic alpha amino acids or the salts thereof may be substituted for those given in accordance with the more general formulas:

P04 (as alkali metal phosphate) grams 5.5 to 3.1.6 C103 (as alkali metal chlorate) do 2.3 to 19.6 P201 (as alkali metal polyphosphate, meta,

pyro, etc.) grams-.. 1.3 to 9.8 P04 (as alkali metal phosphate) do 5.5 to 31.6 C103 (as alkali metal chlorate) do 2.3 to 19.6 Ethylene diamine tetra-acetic acid (as alkali metal salt) "grams" 1.5 to 11.4

During the operation of an alkali metal phosphate solution as a continuous dip process, for example, it has been found that it is necessary to periodically replenish or replace the addition agent of this invention in order to maintain the substantial absence of spot or dust forming substances. The below examples are given to illustrate typically suitable quantities for continuous production and to point out in detail the method of this invention.

Example 1 A concentrated aqueous solution is prepared containing:

P04 (as NaHzPO4) grams/liter-.. 212 P04 (as H3PO4) d0 63 C103 (as NaClOa) do 118 N03v (as NaNOa) do 15 The cleaned sheet steel articles are then immersed in the solution for about 3 minutes at a temperature of about F. to 180 F. and removed. As the solution begins to take on a slightly cloudy appearance, small quantities of tetra sodium ethylene diamine tetra-acetate are added to the solution until it is clear, and similarly the solution is replenished with additional concentrate. Approximately :19 :grams of tetra sodium ethylene diamine tetra-acetate is' required for each 100 square feet of sheet steel processed in order to maintain the solution free of cloudiness and the coating free of spots and dust. All of the panels processed were coated with a gleaming steel blue coating having no trace of dust thereon.

Similar results may be obtained by periodically adding a polyphosphate such as tetra sodium pyrophosphate in an amount of about 14 grams per 100 square feet of panel processed. In order to stabilize the hydrolysis of the polyphosphate chosen for use in the orthophosphate, it is preferable to select the phosphate which are less constant in hydrolysis such as the meta phosphates and polyphosphates when the temperature of operation is relatively low, for example from about 100 F. to about F. When the temperature is somewhat higher and between about 130 F. and about 180 F., and preferably about F., the phosphates which are more stable relative to hydrolysis should be selected such as the pyrophosphates. For maximum uniformity of hydrolysis, it is preferable to admix the meta phosphates and pyrophosphates in accordance with the selected temperature of operation. The particular ratio of meta phosphate to pyrophosphate will vary with the particular conditions of each application, but a few simple tests under operating conditions will quickly establish the optimum quantities. One optimum ratio is illustrated in Example 2.

Example 2 The initial concentrated aqueous solution of Example 1 is diluted with water in the ratio of 24 parts water to 1 part concentrate and the pH of the solution is adjusted with tetra sodium pyrophosphate and disodium pyrophosphate to a pH of 4.0.

Sheet steel panels cleaned as in Example 1 were sprayed with the solution at about 105 F. for one-and-one-half minutes. The spray solution was maintained free of cloudiness by periodically adding about 9 grams P04 in the form of an admixture of 1 part tetrasodium pyrophosphate and 2 parts disodium pyrophosphate for each 100 square feet of panel area sprayed with the solution. The resultant panels were coated with a steel blue coating and did not show any signs of spots or a dusty overlayer. After processing a large number of panels the sediment or sludge formed in the solution and which settled out was measured and found to be about 7.5 grams per 100 square feet of panel area sprayed.

Example 3 In a series of tests, the chlorate of the processing solution of Example 1 was replaced by 3 grams/liter bromat'e, 1 gram/liter nitrite and 3 grams/liter sulfite and the pH adjusted in each case to values varying between 3 and 4.

Preliminarily cleaned steel panels were processed in the solutions and proportions of the addition agents of this invention comparable to those used in Examples 1 and 2, were found to have similar elfects on the coatings formed and on the solutions.

Application of the solutions of Examples l, 2 and 3 to zinc and aluminum surfaces produced adherent, substantially transparent coatings which were highly desirable as a base for paint, and the operating solutions were substantially free of sludge.

It will be appreciated that the above examples are illustrative only and are not intended to limit the scope of the invention to the specific proportions, ingredients or other conditions specified therein. Other polyphosphates may be substituted in Examples 2 and 3 for the pyrophosphates and other polycarboxylie alpha amino acids may be used in Examples 1 and 3 in proportions equivalent to the amounts of the tetra sodium salt there used.

As used in this specification and the appended claims, the term alkali metal is intended in its broad sense to include ammonium.

What is claimed is:

1. An improved alkali metal phosphate solution consisting essentially of an aqueous acidic solution of an alkali metal phosphate and a metallic ion sequestering agent selected from the group consisting of polyphosphates in an amount of about 1 to about 20 grams/liter, and acetic amino acids and the alkali metal salts thereof in an amount equivalent to about 0.77 to about 7.7 grams/liter of ethylene diamine tetra acetic acid, said solution having a pH between 3 and 6.

2. An improved alkali metal phosphate solution consisting essentially of an aqueous acidic solution of an alkali metal phosphate, an oxidizing agent and a metallic ion sequestering agent selected from the group consisting of polyphosphates in an amount of about 1 to about 20 grams/liter, and acetic amino acids and the alkali metal salts thereof in an amount equivalent to about 0.77 to about 7.7 grams/liter of ethylene diamine tetra acetic acid, said solution having a pH between 3 and 6.

3. An improved solution for forming coatings on the surfaces of metals of the group consisting of iron, steel, zinc and aluminum which consists essentially of an aqueous acidic solution of an alkali metal phosphate, an oxidizing agent and a metallic ion sequestering agent selected from the group consisting of polyphosphates and acetic amino acids and the alkali metal salts thereof, said polyphosphate when present being present in an amount of about 1 to 20 grams/liter and said acetic amino acid when present being present in an amount of about .77 to 7.7 grams/liter, said solution having a pH between 3 and 6.

4. A method for forming a coating on the surface of a metal selected from the group consisting of iron, steel, zinc and aluminum which comprises the step of contacting the surface with a solution consisting essentially of an aqueous acidic solution of an alkali metal phosphate and a metallic ion sequestering agent selected from the group consisting of polyphosphates in an amount of about 1 to about 20 grams/liter, and acetic amino acids and the alkali metal salts thereof in an amount equivalent to about 0.77 to about 7.7 grams/liter of ethylene diamine tetra. acetic acid, said solution having a pH between 3 and 6.

5. A method for forming a coating on the surface of a metal selected from the group consisting of iron, steel, zinc and aluminum which comprises the step of contacting the surface with a solution consisting essentially of an aqueous acidic solution of an alkali metal phosphate, an oxidizing agent and a metallic ion sequestering agent selected from the group consisting of polyphosphates in an amount of about 1 to about 20 grams/liter, and acetic amino acids and the alkali metal salts thereof in an amount equivalent to about 0.77 to about 7.7 grams/liter of ethylene diamine tetra acetic acid, said solution having a pH between 3 and 6.

6. A method for forming a coating on the surface of a metal selected from the group consisting of iron, steel, zinc and aluminum which comprises the step of contacting the surface with a solution consisting essentially of an aqueous acidic solution of an alkali metal phosphate, an oxidizing agent and a metallic ion sequestering agent selected from the group consisting of polyphosphates and acetic amino acids and the alkali metal salts thereof, said polyphosphate when present being present in an amount of about 1 to 20 grams/ liter and said acetic amino acid when present being present in an amount of about .77 to 7.7 grams/ liter, said solution having a pH between 3 and 6.

7. A make-up and replenishing material in solid form for alkali metal phosphate solutions which consists essentially of 5.5 to 31.6 grams P04 as alkali metal phosphate, 2.3 to 19.6 grams C10 as alkali metal chlorate and 1.3 to 9.8 grams P207 as alkali metal polyphosphate.

8. A replenishing material in solid form for alkali metal phosphate solutions which consists essentially of 5.5 to 31.6 grams P04 as alkali metal phosphate, 2.3 to 19.6 grams C10 as alkali metal chlorate, and 1.5 to 11.4 grams ethylene diamine tetra-acetic acid as the alkali metal salt thereof.

References Cited in the file of this patent UNITED STATES PATENTS 2,314,887 Lodeesen et al. Mar. 30, 1943 2,318,606 Golbel May 11, 1943 2,332,209 Enquist Oct. 19, 1943 2,437,441 Rogers Mar. 8, 1948 2,481,977 Cinamon Sept. 13, 1949 2,499,261 Rosenbloom Feb. 28, 1950 2,514,941 Drysdale et a1 July 1], 1950 2,528,787 Roland Nov. 7, 1950 2,609,308 Gibson Sept. 2, 1952 FOREIGN PATENTS 599,728 Great Britain Mar. 19, 1948 731,102 Germany Feb. 3, 1943 OTHER REFERENCES Metal Finishing, July 1950, pp. 59-64. 

4. A METHOD FOR FORMING A COATING ON THE SURFACE OF A METAL SELECTED FROM THE GROUP CONSISTING OF IRON, STEEL, ZINC AND ALUMINUM WHICH COMPRISES THE STEP OF CONTACTING THE SURFACE WITH A SOLUTION CONSISTING ESSENTIALLY OF AN AQUEOUS ACIDIC SOLUTION OF AN ALKALI METAL PHOSPHATE AND A METALLIC ION SEQUESTERING AGENT SELECTED FROM THE GROUP CONSISTING OF POLYPHOSPHATES IN AN AMOUNT OF ABOUT 1 TO ABOUT 20 GRAMS/LITER, AND ACETIC AMINO ACIDS AND THE ALKALI METAL SALTS THEREOF IN AN AMOUNT EQUIVALENT TO ABOUT 0.77 TO ABOUT 7.7 GRAMS/LITER OF ETHYLENE DIAMINE TETRA ACETIC ACID, SAID SOLUTION HAVING A PH BETWEEN 3 AND
 6. 